Allow modifier key (Shift/Alt) to drag-and-drop on Multi-Select Grouped Boards

Please bring back drag-and-drop functionality for Board views grouped by Multi-Select Custom Fields.

While disabling it prevents accidental data inflation for standard users, completely locking it down slows down power users. Adding a modifier key (like holding Shift or Alt while dragging) to explicitly “Append Value” would solve both problems: it protects regular users from accidents while restoring a fast, fluid workflow for admins.